The present invention relates to a heat-resistant composite sheet having thermal conductivity and, more particularly, to a thermally conductive heat-resistant silicone rubber composite sheet suitable as a thermally conductive cushion sheet used when a hot pressing process is adopted for forming laminates or flexible boards and for connecting electrodes, such as electrodes of a liquid crystal display, by the use of an electrically conductive anisotropic film. In addition, the invention is concerned with a method of producing the aforesaid silicone rubber composite sheet.
Hitherto known thermally conductive electrical insulating materials are, e.g., silicone rubber in which beryllium oxide powder, aluminum oxide powder, aluminum hydroxide powder, magnesium oxide powder or zinc oxide powder is mixed (as disclosed in Japanese Tokkai Sho 47-32400, wherein the term xe2x80x9cTokkaixe2x80x9d means an xe2x80x9cunexamined published patent applicationxe2x80x9d), and boron nitride-containing silicone rubber which is reinforced by an insulator net (as disclosed in Japanese Jikkai Sho 54-184074, wherein the term xe2x80x9cJikkaixe2x80x9d means an xe2x80x9cunexamined published utility model applicationxe2x80x9d). These silicone rubbers are already used as heat-dissipating insulators for exothermal components, such as power transistors, thyristors, rectifiers, transformers, power MOS and FET. However, those silicone rubbers have a drawback of deteriorating under the influences of impurities in the thermal conductivity imparting agents and their pH values when used under a high temperature condition, specifically 200xc2x0 C. or above.
On the other hand, the foregoing thermally conductive electrical insulating sheets have been employed as cushion sheets in the cases of forming laminates or flexible printed circuit boards by means of a press forming machine, or in the case where an electrically conductive anisotropic film is subjected to hot pressing by means of a pressing machine in order to connect electrode terminals of liquid crystal display and an operation circuit-mounted flexible printed circuit board. For such uses, the glass cloth-reinforced silicone rubber sheet in which boron nitride is mixed is disclosed in Japanese Tokkai Hei 5-198344, and the glass cloth-reinforced antistatic silicone rubber sheet in which both boron nitride and an electrically conductive substance are mixed is disclosed in Japanese Tokkai Hei 6-36853. These cases also have the same deterioration problem of silicone rubber under a high temperature condition as mentioned above.
In recent years, the material properties of flexible printed circuit boards and those of electrically conductive anisotropic films have been changed so as to have suitability for high-temperature forming, while the forming temperature has also been raised for increasing productivity by reduction in a pressing cycle time. Under these circumstances, the heat resistance and the thermal conductivity of a silicone rubber sheet as thermally conductive electrical insulator are of increasing importance. Therefore, Japanese Tokkai Hei 7-11010 proposes the silicone rubber simple sheet in which the carbon black containing volatile components, exclusive of water, in a proportion of at most 0.5 weight % is used as a thermal conductivity providing agent to ensure high heat resistance enough to withstand exposure to temperatures of 300xc2x0 C. or higher and satisfactory thermal conductivity.
However, such an electrical insulating sheet having thermal conductivity is short of strength because it is a simple sheet of silicone rubber, and so there is a fear of breaks by repeated use. In addition, the tackiness of silicone rubber sheet causes another problem that the sheet sticks to the pressure applying tool and the pressed material after hot pressing, and thereby the workability is considerably worsened. Further, when it is used at a temperature of 300xc2x0 C. or above as in the case where electrode terminals of liquid crystal display and an operation circuit-mounted flexible printed circuit board are connected via an electrically conductive anisotropic film in accordance with a hot pressing process using a pressing machine, the silicone rubber sheet causes a problem such that the volatile components present therein pollute the electrode terminals and the operation circuit. In order to avoid such detrimental effects of the silicone rubber sheet, the heating treatment of the sheet at a high temperature and the resin film lamination on the sheet are generally carried out. However, the heating of the laminated sheet up to a high temperature of 150xc2x0 C. or above causes a warp in the sheet because the contraction of the rubber occurs, thereby spoiling the workability.
So our intensive studies have been made to look for thermally conductive heat-resistant silicone rubber composite sheets having not only satisfactory thermal conductivity but also sufficient strength, and further causing no firm adhesion to pressurizing tools by rubber tackiness and ensuring good workability. As a result, it has been found that satisfactory results can be obtained when the silicone rubber sheet in which carbon black containing volatile impurities except water in a content of at most 0.5 weight % and at least one ingredient selected from the group consisting of metals, metal oxides, metal nitrides and metal carbides are mixed in specified proportions is reinforced by a heat-resistant resin film, thereby achieving the present invention.
Therefore, one object of the invention is to provide a thermally conductive heat-resistant silicone rubber composite sheet that has not only high thermal conductivity but also sufficient strength, causes no firm adhesion to pressurizing tools by rubber tackiness and ensures good workability.
Another object of the invention is to provide a method of producing a silicone rubber composite sheet having the aforementioned characteristics.
The foregoing objects of the invention are attained with a composite sheet comprising a silicone rubber sheet and a heat-resistant resin film layer provided on at least one side of the silicone rubber sheet, wherein the silicone rubber sheet is a sheet molded of a silicone rubber composition which comprises (A) 100 parts by weight of organopolysiloxanes having an average polymerization degree of at least 200, (B) 0 to 150 parts by weight of carbon black containing volatile impurities except water in a content of at most 0.5 weight %, (C) 0 to 1,600 parts by weight of at least one thermally conductive substance selected from the group consisting of metals, metal oxides, metal nitrides and metal carbides and (D) a curing agent, provided that the total proportion of the components (B) and (C) is from 10 to 1,600 parts by weight, and having a volatile impurity content of at most 0.2 weight % when it is heated at 150xc2x0 C. for 3 hours; and with a method of producing the aforesaid composite sheet.
In accordance with the invention, a heat-resistant resin film is stuck on the silicone rubber sheet. So the present thermally conductive heat-resistant silicone rubber composite sheet is free of rubber tackiness troubles, has sufficient strength and can ensure good workability. Further, the present composite sheet does not pollute electrode terminals and operation circuits upon hot pressing because the volatile impurity content therein can be reduced to 0.2 weight % or below. In addition, the carbon black incorporated in the present composite sheet prevents electrostatic buildup to avoid the adhesion of dust to the composite sheet in use and further inhibit electronic components loaded on circuits from causing a breakdown by electric discharge.
The organopolysiloxanes as Component (A) of the present silicone rubber composition have an average polymerization degree of at least 200, preferably from 3,000 to 20,000, and are represented by the average composition formula RnSiO(4-n)/2, wherein n is a positive number of from 1.95 to 2.05 and R represents a substituted or unsubstituted monovalent hydrocarbon group. Examples of such a monovalent hydrocarbon group include an alkyl group, such as methyl, ethyl or propyl group, a cycloalkyl group, such as cyclopentyl or cyclohexyl group, an alkenyl group, such as vinyl or allyl group, an aryl group, such as phenyl or tollyl group, and halogenated hydrocarbon groups, such as the groups recited above whose hydrogen atoms are partly replaced by chlorine or fluorine atoms. When 0.001 to 5 mole %, especially 0.01 to 1 mole %, of R are alkenyl groups, the resultant organopolysiloxanes can have good effects.
In general the main chain of organopolysiloxanes suitable for the invention is constituted of dimethylsiloxane units alone or dimethylsiloxane units modified in parts by introduction of vinyl, phenyl or trifluoropropyl groups. Further, it is advantageous that the molecular chain ends of the present organopolysiloxanes be blocked with triorganosilyl groups, such as trimethylsilyl, dimethylvinylsilyl or trivinylsilyl groups, or hydroxyl groups. In addition, the organopolysiloxanes suitable for Component (A) are those having viscosity of at least 300 cs at 25xc2x0 C.
In cases where the organopolysiloxanes having an average polymerization degree lower than 200 are used as Component (A), the cured composition is inferior in mechanical strength, and the rubber sheet made therefrom is easily broken.
The carbon black as Component (B) not only raises heat resistance, thermal conductivity and mechanical strength of the composite sheet, but also makes the silicone rubber sheet electrically conductive to impart antistatic properties thereto. The content of volatile impurities except water in carbon black usable as Component (B) of the invention is at most 0.5 weight %, preferably at most 0.4 weight %. While the carbon black products are generally classified by their production methods into furnace black, channel black, thermal black and acetylene black, the acetylene black and the electrically conductive carbon black disclosed in Japanese Tokkai Hei 1-272667 are suitable for the present carbon black having a volatile impurity content of at most 0.5 weight %.
As a method of measuring the volatile impurity content as mentioned above, the method described in JIS K 6221 as xe2x80x9cMethod of testing carbon black for rubber usexe2x80x9d is adopted in the invention. More specifically, a prescribed amount of carbon black is placed in a crucible, heated at 950xc2x0 C. for 7 minutes, and then the evaporation loss is measured.
The suitable proportion of Component (B) mixed is from 0 to 150 parts by weights, preferably from 10 to 100 parts by weight, particularly preferably from 20 to 80 parts by weight, to 100 parts by weight of Component (A). When the proportion of Component (B) used is increased beyond 150 parts by weight, it becomes difficult to mix Component (B) homogeneously with the other components and the moldability of the resulting composition is reduced.
Component (C) is at least one substance selected from the group consisting of metals, metal oxides, metal nitrides and metal carbides, and functions so as to impart thermal conductivity to the present silicone rubber sheet. Examples of such a substance include silver powder, copper powder, iron powder, nickel powder, zinc oxide, magnesium oxide, aluminum oxide, silicon oxide, iron oxides, boron nitride, aluminum nitride, silicon nitride, silicon carbide and boron carbide.
The suitable proportion of Component (C) mixed is from 0 to 1,600 parts by weight, preferably from 0 to 1,200 parts by weight, to 100 parts by weight of Component (A). When the proportion of Component (C) used is increased beyond 1,600 parts by weight, it becomes difficult to mix Component (C) homogeneously with the other components and the moldability of the resulting composition is reduced.
The total proportion of Components (B) and (C) in the present composition is required to be from 10 to 1,600 parts by weight, preferably from 40 to 1,200 parts by weight, particularly preferably from 45 to 1,000 parts by weight. In a case of placing importance on the heat resistance of silicone rubber, it is advantageous to raise the proportion of carbon black used.
The curing agent as Component (D) can be selected properly from conventional curing agents for silicone rubber use. Examples of a curing agent suitable for Component (D) include organic peroxides such as di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and dicumyl peroxide, which are suitable for cure by radical reaction; curing agents suitable for cure by addition reaction, such as the combination of organohydrogenpolysiloxanes containing per molecule at least two hydrogen atoms directly attached to silicon atoms with a platinum catalyst in a case where the organopolysiloxanes as Component (A) contain at least two alkenyl groups per molecule; and curing agents suitable for cure by condensation reaction, such as organosilicone compounds containing at least two hydrolyzable groups per molecule, specifically at least two alkoxy, acetoxy, ketoxime or propenoxy groups per molecule, in a case where the organopolysiloxanes used as Component (A) contain at least two silanol groups per molecules.
In adding the curing agent as recited above, the amount thereof may be in the same range as adopted in usual curing of silicone rubber compositions. Specifically, the suitable amount of organic peroxide added in radical reaction is from 0.1 to 10 parts by weight per 100 parts by weight of Component (A); while, in the case of addition reaction, it is effective to use organohydrogenpolysiloxanes in such an amount that the proportion of their SiH groups to the alkenyl groups in Component (A) is from 0.5 to 5 mole % and a platinum catalyst in an amount of 1 to 2,000 ppm.
The heat resistance of the present silicone rubber composition can further be raised by the addition of cerium oxide powder. The suitable amount of cerium oxide powder added is from 0.1 to 5 parts by weight per 100 parts by weight of Component (A). When the amount added is increased beyond 5 parts by weight, the heat resistance is sometimes lowered by contraries. Additionally, it is beneficial to use cerium powder having relatively large specific surface area of at least 50 m2/g, measured by BET method.
To the silicone rubber composition used in the invention, various additives may be added, if desired. Examples of such additives include a filler such as clay, calcium carbonate or diatomaceous earth, a dispersing agent such as a low molecular siloxane ester or a silanol group-containing low molecular organosiloxane, an adhesiveness-imparting agent such as a silane coupling agent or a titanium coupling agent, an incombustibility-imparting platinum group metal catalyst, and poly(tetrafluoroethylene) particles for increasing the Green strength of rubber compound. Additionally, the present silicone rubber composition can be prepared by kneading the ingredients as recited above by means of a mixing machine, such as a two-rod roll, a kneader, a Banbury mixer or a planetary mixer. As to the curing agent, however, the suitable addition time therefor is just before the use of the composition.
The heat-resistant resin film used in the invention is required to have high mechanical strength and excellent releasability at high temperatures, because the present thermally conductive silicone rubber composite sheet is used even at temperatures of around 300xc2x0 C. Therefore, the resin films having glass transition points of 200xc2x0 C. or higher, such as aromatic polyimide films, polyamideimide films, aromatic polyamide films, polyether sulfone films and polyether imide films, and fluorine-contained resin films having melting points of 300xc2x0 C. or higher, such as polytetrafluoroethylene (PTFE) film and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) film, can be utilized as heat-resistant resin films for the invention.
As to examples of commercially available heat-resistant resin films, Capton (trade name, a product of Toray Du Pont Co., Ltd.), Apical (trade name, a product of Kanegafuchi Chemical Industry Co., Ltd.) and Eupilex (trade name, a product of Ube Industries, Ltd.) are on the market as aromatic polyimide film; Aramica (trade name, a product of Asahi Chemical Industry Co., Ltd.) is on the market as organic polyamide film; and Teflon (trade name, a product of Du Pont Japan Limited) and Nitoflon (trade name, a product of Nitto Electric Industrial Co., Ltd.) are on the market as fluorine-contained resin.
In particular, it is favorable to reinforce the heat-resistant resin film with glass cloth from the viewpoint of mechanical strength.
Further, it is advantageous to impart electric conductivity to the heat-resistant resin film as recited above by mixing carbon black in the resin, or impart thermal conductivity thereto by mixing therein a thermally conductive powder such as aluminum oxide powder or magnesium oxide powder. As to the thermal conductivity-imparted heat-resistant resin film, Capton MT (trade name, a product of Toray Du Pont Co., Lltd.) is on the market.
The suitable thickness of the present heat-resistant resin film is from 5 to 300 xcexcm, particularly from 10 to 100 xcexcm. When the thickness of resin film is too small, the film itself is insufficient in mechanical strength, so there is a fear that the film is broken at the time when the sheet is molded or used for hot pressing; while, when the thickness is too great, the transmission of heat through the film is retarded, so that satisfactory hot pressing cannot be achieved.
In molding the present silicone rubber composition into silicone rubber sheets, the following two methods can be adopted: One method comprises pushing out a silicone rubber composition containing up to a curing agent from a calender or extruder to shape the composition into a sheet of the desired thickness, and then curing them by heating; and the other method comprises coating a carrier film with a liquid silicone composition or a silicone rubber composition dissolved in a solvent such as toluene, curing the composition, and then peeling the cured composition from the carrier film. In order to achieve the volatile impurity content of 0.2 weight % at the most, preferably 0.1 weight % at the most, in the present silicone rubber sheet at the time when heated at 150xc2x0 C. for 3 hours, the foregoing cured sheet or coating is desirably subjected to heat treatment. To the invention, it is advantageous to place the molded sheets as mentioned above in a dryer or continuous furnace and subject them to heat treatment at a temperature of at least 150xc2x0 C.
In bonding the thus prepared silicone rubber sheet to a heat-resistant resin film, it is effective to previously apply a primer to the heat-resistant film. When the heat-resistant resin film used is a fluorine-contained resin film, it is favorable to impart adhesiveness to the film in advance by etching the film with, e.g., a sodium-naphthalene solution.
In order to make the silicone rubber sheet adhere effectively to the heat-resistant resin film, it is advantageous to use an adhesive, especially a silicone adhesive. Suitable examples of a silicone adhesive include adhesion aid-added single-liquid, double-liquid or triple-liquid type silicone rubber and silicone varnish of the type which has tackiness in a fresh stage and comes to have adhesiveness by storage at room temperature or heating. From the viewpoints of adhesiveness and coatability, silicone varnish for adhesion use is preferred in particular.
For making the present composite sheet, it is favorable to coat a silicone adhesive on a heat-resistant resin film such as an aromatic polyimide, polyamideimide or PTFE film, preferably in a thickness of 10 to 50 xcexcm, and then unite the resin film and the present silicone rubber sheet into a laminate after the adhesive is dried or as it remains wet.
The suitable thickness of the thus formed silicone rubber composite sheet is from 0.1 to 10 mm. When the composite sheet has a too small thickness, it sometimes fails in ensuring uniform application of pressure to a material to be pressed in a hot pressing process because of its poor response to the unevenness on the material; while, when the thickness is too great, the resultant sheet retards the transmission of heat in some cases. Additionally, the heat-resistant resin film can be provided on not only one side but also both sides of the present silicone rubber sheet depending on the desired purposes.